Hot Nights: July Shatters Temperature Records

July was hot: Washington, D.C., Oklahoma City, Philadelphia, and Austin set records for not just their warmest July in history, but their warmest month on record. The heat prompted people to hide indoors, crank up the air conditioning, or attempt stunts such as cooking eggs on the roof. But what made this month unusual wasn't only the hot days, but rather the hot nights.

Even though repeat heat waves brought sizzling hot days, overnight temperatures broke far more records: According to the National Climatic Data Center (NCDC), in July there were 6,106 record high minimum temperatures, and "only" 2,722 record high daytime temperatures.

Curious about these numbers, I looked more into the past decade of temperature records, and also spoke with Climate Central’s staff scientists. Have nights generally warmed faster than days? And if so, why?

I downloaded data from NCDC's database of more than 5,000 weather stations across the United States. For each day since January 1, 2000, I looked at four possible records — two for the nighttime low temperatures (record low minimum and record high minimum), and two for the daytime high temperatures (record high maximum and record low maximum).

Since 2000, in the average month, record highs (high maximum temperature) beat out record lows (low minimum temperature) by a two to one margin. This is exactly what has been found in previous peer reviewed studies — including this study, published in Geophysical Research Letters in 2009, by Climate Central’s Claudia Tebaldi and three other researchers.
But looking at these four types of records, it appears that nights have warmed even more: the average month recorded 10 percent more record high minimum temperatures than record high maximums. The record low temperatures tell an even more compelling story: it was much more likely for the daytime temperature to be colder than average than it was for the nighttime ones. There were only 1,235 record low minimum temperatures set per month, while there were 1,697 record low maximum temperatures set per month. By this measure, a record cold day was 40 percent more likely than a record cold night.
Interestingly, when I looked at different months of the year, I found that the nighttime warming was even greater during the summer. During June, July and August, record high minimum temperatures outnumbered record low minimum temperatures by about three to one.
All of this is all a fancy way of saying “nights have warmed more than days.”
I spoke with Phil Duffy, Climate Central’s chief scientist, about why nighttime lows are warming faster than the daytime highs. He replied that the answer isn’t straightforward, and then he referred me to research that has shown that an increase in cloudiness (as well as a few other factors) has warmed nights more than days. During the day, clouds both warm and cool, as they act like a blanket to reflect heat back to the surface (warming), but they also reflect sunlight back to space (cooling). At night, they only warm temperatures, acting like an insulating blanket. Thus, nights warm more than the days, and this is exactly what climate models predict. In fact, this is a good example of climate models making a prediction (warmer nights), and then having the prediction born out by the data.
We will come back to this issue in future weeks, as we compile more data on nighttime temperatures and examine climate model projections more closely.
Below is an info-graphic that lets you look at these four types of records for any month since January 2000. Notice that there is a significant amount of variation between months, but that warm temperature records, and especially record high nighttime temperatures, outnumber the cold temperature records. (I also note on the graphic how many possible records there were in a given month, which is roughly the number of stations times the number of days in the month.) All of the data in the graphic comes from NCDC.